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Treasure Finder 

Make] Projects 

Treasure Finder 

Written By: Steve Hobley 



3/4" Hole Cuttinq Bit (1) 


1/4" plywood (1) 

Drill and drill bits (1) 


3/4" hardwood dowel (3-4' long) (1) 

Hacksaw (1) 


1/2" hardwood dowel (2" lonq") (1) 

Iron, electric (1) 


1/4" hardwood dowel (4" lona) (1) 

Laser printer (1) 


Threaded PVC pipe connector (1/2" 

Miter box (1) 

bore) (1) 

for 1/4" plvwood. 


Lock nut. brass. 1/2" pipe (1) 

Multimeter with frequency counter (1) 

Copper clad PC board, sinale-sided 4 

from RadioShack. Or you can use an 

1/2" x 6 1/8" (1) 



Resistor Assortment. 1/8 watt. 500-piece 

PCB Etchant (1) 


from RadioShack. 

from RadioShack. 

Sandpaper (1) 


Capacitor. 0.1 F (5) 


from RadioShack. 

for 1/4" plvwood. 


Capacitor. 0.01 F (5) 

Solder, rosin core (1) 

from RadioShack. 

from RadioShack. 


Capacitor. 220 F electrolytic (2) 

Solderinq Iron. 15 Watt (1) 

from RadioShack. 

from RadioShack. 


Magnet wire. 26 gauge (1) 
from RadioShack. 

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Treasure Finder 

Transistor. NPN. 2N3904 (6) 

from RadioShack. 

Wire, hookup. 22 gauge stranded (1) 

from RadioShack. 

Speaker wire. 4' (1) 

from RadioShack. 

Speaker, small 8Q(1) 

from RadioShack. 

Switch. SPST toggled) 

from RadioShack. 

Potentiometer. 5kQ. audio taper (1) 

from RadioShack. 

Audio jack, 1/8". mono, panel mount (1) 

Headphones, 1/8" plug, mono or stereo 


from RadioShack. 

Battery, 9V (1) 
from RadioShack. 

Snap connector for 9V battery (1) 
from RadioShack. 
Wood glued) 
Epoxy (1) 


Who hasn't dreamed about owning their own metal detector and searching for buried 
treasure, or at least a few dropped coins? In this project, we will build a metal detector 
based on a dual oscillator circuit. One oscillator is fixed, and the other varies depending 
upon the proximity of metal objects. The beat frequency between these two oscillator 
frequencies is in the audible range, and as the detector passes over metal objects, you will 
hear a shift in this beat frequency. Different metal types will cause a positive or negative 
shift, raising or lowering the audible frequency. 
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Treasure Finder 

This "heterodyning" principle is used in many applications - radio, scientific measurements, 
even the Theremin! 

Check out more Weekend Projects . 

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Treasure Finder 

Step 1 — Make the Printed Circuit Board 

• Download the PCB design from 
here (credit: 

• Print the design out on a laser 
printer, then etch it onto plain 
copper clad board using the Toner 
Transfer method. Here's a good 
page on the technique. 

• With Toner Transfer, you print a 
mirror-image of the board design 
using an ordinary laser printer, 
then transfer the pattern onto the 
copper cladding using an electric 
iron. During the etching step, the 
toner acts as a mask, keeping 
the copper traces underneath 
while the rest of the copper 
dissolves away in a chemical 

• If you don't want to do any 
etching, you can build the circuit 
by stringing it up with wires on 
plain perf board-- although this 
won't be as neat. 

• I made my board approximately 
twice as large as it needs to be. 
This was due to a print scaling 
error on my part, but the board will 
still work just fine. 

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Treasure Finder 

Step 2 — Populate the board - transistors and electrolytic caps 

• Begin by soldering in the 6 NPN transistors. 

• Note the orientation of the transistors. Refer to the transistor packaging or datasheet to 
confirm their correct orientation -- specifically, which pins are the collector (C) and 
emitter (E). Different transistors have different pin configurations. (The base pin (B) is 
almost always in the middle.) 

• I backlit all of the circuit board assembly photographs - just to make it clearer 
where each part goes. 

• Next, add the two 220 F capacitors -- these are polarized, and must only go in one way 
'round. Look for a stripe on the capacitor, with marks the cathode (-) side. 

• Unlike most schematics, the diagram here actually matches the layout of the PCB. 


Step 3 — Populate the board (cont'd) - caps and resistors 

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• After the big capacitors, add five 0.1 F polyester capacitors in the locations shown. 
These are not polarized and can go in either way. 

• Add the five smaller 0.01 F capacitors - again, these can go in either way 'round. 

• Start adding the resistors - first were six 10KQ resistors (Brown, Black, Orange, Gold). 
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Treasure Finder 

Step 4 — Populate the board (cont'd) - resistors and offboard leads 

• Add the 2.2MQ (Red, Red, Green, Gold) and the two 39KQ (Orange, White, Orange, Gold) 

• The last resistor is a 1KQ (Brown, Black, Red, Gold). 

• Finally, add wire pairs for power (red/black), audio out (green/green), reference coil 
(black/black), and detector coil (yellow/yellow). 

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Treasure Finder 

Step 5 — Wind the reference coil 

• Next up is winding the two coils that are part of the LC oscillator circuit . The first coil is the 
reference (or tuning) coil. I used #26 wire for this. 

• Cut a piece of dowel (about 1/2 diameter and 2" long). Drill three holes in the dowel to pass 
the wire through: one lengthwise through the middle of the dowel, and two more 
perpendicular at each end. 

• Slowly and carefully wind as many turns of wire as you can around the dowel in a single 
layer. Leave 1/8" of bare wood each end. 

• TIP: Put the spool of wire on the floor with some unused leftover dowel through 
it. Rest your feet on either end of the spool dowel and apply gentle pressure to 
adjust the spool's tension as you dispense and wind more wire. 

• Resist the temptation to "twist" the wire as you wind - this is the most intuitive 
way of winding, but it's the wrong way. You should rotate the dowel and let it pull 
the wire from the spool. 

• Thread each wire end through the perpendicular holes in the dowel, and then one of them 
through the lengthwise hole. Secure the wire with tape once you are done. 

• Finally, use some 100-grit sandpaper to remove the coating on the two exposed ends of 
the coil wire. 


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Treasure Finder 

Step 6 — Make the detector coil 

• Cut a coil holder from 1/4" plywood. I've included a template you can print out and use as a 
cutting guide. 

# Sandwiching the smaller ring between two larger ones creates the groove to wind the 
detector coil in. 

• Using the same #26 wire as the reference coil, wind 10 turns around the groove. My coil 
has a diameter of 6". 

• Use a 1/4" wooden dowel peg to attach the handle to the holder. Don't use a metal 
bolt, or you'll be detecting treasure everywhere! 

• Again, use 100-grit sandpaper to remove the coating on the wire ends. 


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Treasure Finder 

Step 7 — Tune the reference coil 


• Now we need to tune the frequency of the reference coil in our circuit to 100 kHz, by 
gradually shortening it and measuring its frequency. 

• I used an oscilloscope to do this, but many multimeters have a frequency 
counter and this will do the job just as well. 

• Start by connecting the coil into the circuit and connecting power. Connect probes from a 
'scope or multimeter to both ends of the coil and measure its frequency. It should be less 
than 100 kHz. 

• Unwind some wire to shorten the coil. This reduces the coil's inductance, raising its 
frequency. Then retest. Repeat until it's at about 100 kHz. 

• Once I was done, my coil measured 1.23 inches in length. 


Here is a video of me unwinding turns on the coil until the scope read 100 kHz. 

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Treasure Finder 

Step 8 — Build the electronics enclosure 

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• Cut the pieces for a simple 3/4 box from plywood (a template is included in the project 
files). Assemble and glue the box together. 

• My enclosure needed to be bigger than normal because I made the circuit board 
too large. 

• Ignore the pattern "etched" into the back plane template. This was my original 
idea for a reference coil mount, but instead, fitting it into PVC pipe was too clever 
to pass up. 

• Drill one hole in the box to hold a 1/2" PVC pipe fitting that will carry the reference coil, and 
another hole for the speaker. 

• Epoxy the PVC fitting into the box and let it set. 

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Treasure Finder 

Step 9 — Put it all together 

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• Tuck the reference coil inside the PVC and screw the brass nut onto the outer threads. 
Attach it (black wires) to the PCB. 

• TIP: If the nut or coil comes loose, try wrapping some Teflon tape (available from 
the plumbing section of the hardware store) around the threads. 

• Cut a 3' length of 3/4" hardwood dowel for the handle. Drill a 1/4" hole through one end and 
connect the detector coil with a 1/4" dowel peg. Thread the two large holes in the 
electronics enclosure over the handle. 

• Loosely wrap some some speaker wire around the handle and connect it between the 
detector coil and the PCB (yellow wires). 

• I hooked up the 80 speaker to the green output wires and a 9V battery to the power wires. 
Be careful of the polarity: red is (+) and black is (-). 

• I also wired in a switch on the red wire, which makes it much easier to turn on 
and off! 

• And last, but by no means least, you need to adjust the brass nut on the reference coil to 
create the right kind of tone. Here's a video that explains this . 

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Treasure Finder 

Step 10 — Improvements - volume control and headphone jack 

Adding a volume control and headphone jack 


• To improve the "human factors" of your metal detector, add a volume control. The 
screeching and squealing that this device can make can be "upsetting" for anyone that 
might be around you. 

• Even better, add a headphone jack! Then you can use your treasure finder anywhere 
without disturbing others. I had to drill a couple more holes into the case to fit these. 

• I've included a schematic for the additional parts, which you add onto the two speaker 
connections. If you wire up the jack correctly it will mute the speaker when you plug the 
headphones in. 

This project uses a phenomenon called "heterodyning" to combine two high-frequency signals to 
generate a much lower-frequency audio signal. Radio communication, precision rangefinders, 
and the Theremin all use this principle. You can learn more about heterodyning here . 

Note: The idea of putting a small coil inside of a PVC pipe and tuning it by turning a brass nut is 
brilliant - I will definitely be using this in future projects. 

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